1. Technical Field of the Invention
The present invention relates generally to fuel supply systems for supplying fuel to the fuel injectors on an internal combustion engine and more particularly to a model predictive control closed-loop controller for an electronically controlled fuel supply system.
2. Background of the Invention
In the quest for high efficiency emissions-free internal combustion engines, engine manufacturers have attempted to develop engine fuel supply systems that achieve complete fuel combustion. One such system is the high pressure ignition (HPI) fuel system. Such systems require each fuel injector to achieve SAC pressures in excess of 30,000 psi during the injection cycle. In order to make such injection pressures possible, precise supply and control of both fuel and timing fluid to each engine fuel injector is required. In an HPI fuel system, the two most important system variables which need to be precisely controlled are the rail (injector fuel supply) and timing pressures.
Several HPI fuel supply systems are disclosed in U.S. Pat. No. 4,971,016 issued to Peters et al., which is assigned to the assignee of the present application and incorporated herein in its entirety by reference. One such HPI fuel supply system is illustrated in FIG. 1 and indicated generally at 10. Fuel supply system 10 is a basic closed-loop electronically controlled injector fuel supply system. A gear pump 12 pumps fuel from a reservoir 14 into a fuel supply channel 16. A pressure regulator 18 regulates the pressure of the fuel in supply channel 16 as required before supply channel 16 bifurcates to form a fueling channel 20 and a timing fluid channel 22. A pulse width modulated solenoid pilot valve 24 is positioned on one side of a servo valve 26 in section 28 of the fueling channel 20, and a similar pulse width modulated solenoid pilot valve 30 is positioned on one side of a servo valve 32 in section 34 of the timing channel 22. Each of the solenoid valves is also connected to a drain line which ultimately provides a fluid connection between the valve and fluid reservoir 14. Drain line 36 allows fluid to flow from solenoid valve 24 to reservoir 14, and drain line 38 allows fluid to flow from solenoid valve 30 to reservoir 14. The fuel in reservoir lines 36 and 38 will have a lower pressure khan the fuel in lines 28 and 34. Restricted orifices 40 and 42 are positioned in channel sections 28 and 34, respectively, to assist in maintaining the appropriate pressure levels in these lines by restricting the flow of fuel past each of the servo valves. A pressure transducer 44 downstream of the servo valve 26 measures the pressure of the fuel in fuel channel 46, while a similar pressure transducer 48 measures the pressure of the timing fluid/fuel in channel 50. These pressure measurements are transmitted to an electronic control unit 52 (ECU). This electronic control unit, in its most basic form, requires only an engine throttle position signal 54 and an engine speed (RPM) signal 56 as input to control the pressure of the fuel in the channels 46 and 50. A lookup table (not shown) is included in ECU 52. The desired fuel and timing fluid pressures for specific engine throttle positions and engine speeds are programmed into the lookup table so that when the ECU 52 receives pressure information from the pressure transducers 44, 48, this pressure information is compared with the desired pressures for the specific engine throttle position and engine RPM and the ECU 52 transmits signals to the solenoid valves 24, 30, to set them accordingly. The position of the solenoid valve 30 will control the amount of timing fluid supplied to the injector and, therefore, the pressure of the timing fluid and the advance of timing. Similarly, the position of the solenoid valve 24 will control the amount of fuel supplied to the injector fuel rail and, therefore, the fuel pressure. Because this system includes a feedback signal indicative of the pressure produced by the control signal, it is referred to as a closed-loop control.
Therefore, information regarding engine operating conditions is used to provide an output signal to each of the valves 24, 30 that will actuate each valve to set an appropriate pilot pressure on one side of each of the servo valves 26 and 32. The servo valve 26 in fueling channel 20 will then regulate the fuel flow until the pressure of the fuel flowing into fueling channel 46 is equal to the pilot pressure. The fuel in channel section 46 is then supplied at this pressure to a plurality of injector fuel passages, shown schematically at 58. The pressure of the fuel to be injected, therefore, is precisely controlled by the servo valve 26 in accordance with the pilot pressure set by the solenoid valve 24. Likewise, the servo valve in timing fluid channel 22 regulates the flow of timing fluid (fuel) into timing fluid channel section 50 so that the pressure of the timing fluid supplied to the injectors, shown schematically at 59, is equal to the pilot pressure set by the solenoid valve 30.
Referring now to FIG. 2, there is indicated generally at 70 a prior art proportional-integral-derivative (PID) control structure, which may be implemented in the ECU 52 of FIG. 1, for example. Due to the noise in the pressure measurements from pressure transducers 44 and 48, the derivative portion of the algorithm is normally turned off. Therefore, the control structure 70 is practically a PI controller. The desired pressure 72 is calculated by the ECU 52 based upon throttle position 54 and engine speed 56. The actual pressure 74, as measured by either the pressure transducer 44 or 48, is also input to the PID algorithm 78 via the feedback path 76. Based upon a comparison of the desired pressure 72 and the fedback actual pressure 74, the PID algorithm 78 determines a commanded pressure 80. The commanded pressure 80 is chosen with the intent of causing the actual pressure 74 to equate with the desired pressure 72. The commanded pressure 80 is applied to a lookup table 82 which determines an appropriate duty cycle 84 to be applied to the respective pilot valve 24 or 30. The block 86 represents the system transfer function such that application of the duty cycle signal 84 to the system produces a pressure in the system as represented by the actual pressure 74. However, the actual pressure 74 as measured by pressure transducers 44 and 48, is subject to a noise component 88 which is associated with the firing frequency (and multiples and sub-multiples of it) plus any other system disturbances (such as gear pump pressure variation effects, timing fluid/injector fuel loop interactions, etc.). In order to counteract this noise component 88, prior art control systems such as the system 70 incorporate a filter 90 in the feedback path 76. The filter 90 is a 4-pole averaging filter, which effectively removes any noise component 88 at the firing frequency and multiples of this frequency, if pressure measurements are synchronized with engine speed measurements. However, the filter 90 cannot remove any sub-multiples of the firing frequency noise components in the pressure measurements.
This inability of the prior art control structures to completely filter the noise component 88 creates serious problems in engine performance when a fast response time is required. For example, it is sometimes desirable to have a response time for a step-input to the fuel and timing pressure loops on the order of 30 to 50 milliseconds. FIG. 3 shows actual fuel rail pressure response obtained from a six-cylinder Cummins L10 engine controlled by the controller 70 of FIG. 2 using the parameters of Kpl=1.1 and KI=0.6. These parameters give a response time to a step input of approximately 50 milliseconds. This test was conducted at 600 RPM with 100-500 ft-lbs. torque changes. Although it is possible to tune the fuel rail pressure loop for fast system response, as demonstrated by the data in FIG. 3, it is not feasible in practice since timing stability is totally lost when the control parameters are selected for fast response. In other words, the faster the system response, the worse the timing stability becomes. There is therefore a need in the prior art for a fuel system control structure which allows for a fast response time as well as providing for timing stability. The present invention is directed towards meeting those needs.